A **hue** refers to the gradation of color within the optical spectrum, or visible spectrum, of light. "Hue" may also refer to a particular color within this spectrum, as defined by its dominant wavelength, or the central tendency of its combined wavelengths. For example, a light wave with a central tendency within 565-590 nm will be yellow.

In painting color theory, a **hue** refers to a *pure* color —one without added white (tint) or black (shade) pigment.

In an RGB color space, **hue** can be thought of as an angle *φ* in standard position. To calculate *φ*, let *R*, *G*, *B* be the color coordinates in RGB space, defined on a scale from zero to one. Then, after obtaining the brightness *μ* and the saturation *σ*, the hue could be obtained from

- $ \phi = \arccos \left( {R - \mu \over \sigma \sqrt{2}} \right) $

(Compare with standard score). Using this formula, *φ=0* (in radians) would correspond to red, while *φ=2π/3* would correspond to blue, and *φ=4π/3* would correspond to green.

The RGB coordinates should be derivable from the *μ*, *σ*, *φ* coordinates as follows:

- $ R = \mu + \sigma \sqrt{2} \cos \phi, $
- $ G = \mu + \sigma \sqrt{2} \cos \left( \phi + {4 \pi \over 3} \right), $
- $ B = \mu + \sigma \sqrt{2} \cos \left( \phi + {2 \pi \over 3} \right). $

Hue is a coordinate (an angle of rotation) in HSL color space and HSV color space.

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